Single crystal photovoltaic devices, especially silicon photovoltaic devices, have been utilized for some time as sources of electrical power because they are inherently non-polluting, silent, and consume no expendable natural resources in their operation. However, the utility of such devices is limited by problems associated with the manufacture thereof. More particularly, single crystal materials (1) are difficult to produce in sizes substantially larger than several inches in diameter, (2) are thicker and heavier than their amorphous counterparts; and (3) are expensive and time consuming to fabricate.
Recently, considerable efforts have been made to develop processes for depositing amorphous semiconductor films, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n-type devices substantially equivalent to those produced by their crystalline counterparts. It is to be noted that the term "amorphous," as used herein, includes all materials or alloys which have long range disorder, although they may have short or intermediate range order or even contain some crystalline inclusions.
For many years, such work with amorphous silicon or germanium films was substantially unproductive because of the presence therein of microvoids and dangling bonds which produce a high density of localized states in the energy gap. Initially, the reduction of the localized states was accomplished by glow discharge deposition of amorphous silicon films wherein silane (SiH.sub.4) gas is passed through a reaction tube where the gas is decomposed by a radio frequency (r.f.) glow discharge and deposited on a substrate at a substrate temperature of about 500-600 degrees K. (227-327 degrees C.). The material so deposited on the substrate is an intrinsic amorphous material consisting of silicon and hydrogen. To produce a doped amorphous material, phosphine gas (PH.sub.3), for n-type conduction, or diborane (B.sub.2 H.sub.6) gas, for P-type conduction is premixed with the silane gas and passed through the glow discharge reaction tube under the same operating conditions. The material so deposited includes supposedly substitutional phosphorus or boron dopants and is shown to be extrinsic and of n or p conduction type. The hydrogen in the silane was found to combine, at an optimum temperature, with many of the dangling bonds of the silicon during the glow discharge deposition to substantially reduce the density of the localized states in the energy gap, thereby causing the amorphous material to more nearly approximate the corresponding crystalline material.
It is now possible to prepare by glow discharge or vapor deposition thin film amorphous silicon alloys in large areas that possess acceptable concentrations of localized states in the energy gaps thereof, and high quality electronic properties. Suitable techniques are fully described in U.S. Pat. No. 4,226,898, entitled "Amorphous Semiconductors Equivalent to Crystalline Semiconductors," issued in the names of Stanford R. Ovshinsky and Arun Madan on issued Oct. 7, 1980 and in U.S. Pat. No. 4,217,374, under the same title to Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980 and U.S. Pat. No. 4,517,223, issued May 14, 1985 in the names of Stanford R. Ovshinsky, David D. Allred, Lee Walter and Stephen J. Hudgens entitled "Method of Making Amorphous Semiconductor Alloys and Devices Using Microwave Energy." As disclosed in these patents, it is believed that fluorine introduced into the amorphous silicon semiconductor operates to substantially reduce the density of the localized states therein and facilitates the addition of other alloying materials, such as germanium.
Activated fluorine is believed to readily diffuse into, and bond to, amorphous silicon in a matrix body to substantially decrease the density of localized defect states therein. This is thought to be due to the fact that the small size of the fluorine atoms enables them to be readily introduced into an amorphous silicon matrix. The fluorine therefore bonds to the dangling bonds of the silicon and forms a partially ionic stable bond with flexible bonding angles, which results in a more stable and more efficient compensation or alteration than could be formed by hydrogen, or other compensating or altering agents which were previously employed. Fluorine is considered to be a more efficient compensating or altering element than hydrogen when employed alone or with hydrogen, because of its exceedingly small size, high reactivity, specificity in chemical bonding, and highest electronegativity.
Compensation may be achieved with fluorine, alone or in combination with hydrogen, upon the addition of such element(s) in very small quantities (e.g., fractions of one atomic percent). However, the amounts of fluorine and hydrogen most desirably used are much greater than such small percentages, permitting the elements to form a silicon-hydrogen-fluorine alloy. Thus, alloying amounts of fluorine and hydrogen may, for example, be used in a range of 0.1 to 5 percent or greater. The alloy thus formed has a lower density of defect states in the energy gap than can be achieved by the mere neutralization of dangling bonds and similar defect states. In particular, it appears that use of larger amounts of fluorine participates substantially in effecting a new structural configuration of an amorphous silicon-containing material and facilitates the addition of other alloying materials, such as germanium. Fluorine, in addition to the aforementioned characteristics, is believed to (1) be an organizer of local structure in the silicon-containing alloy through inductive and ionic effects, and also (2) influence the bonding of hydrogen by acting to decrease the density of the defect states which hydrogen normally contributes. The ionic role that fluorine plays in such an alloy is an important factor in terms of the nearest neighbor relationships.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was described at least as early as 1955 by E. D. Jackson, U.S. Pat. No. 2,949,498, issued Aug. 16, 1960. The multiple cell structures therein disclosed utilized p-n junction crystalline semiconductor devices. Essentially, the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc.). The tandem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by a smaller band gap material to absorb the light passed through the first cell or layer. By substantially matching the generated currents from each cell, the overall open circuit voltage is the sum of the open circuit voltage of each cell while the short circuit current remains substantially constant.
Many publications on crystalline stacked cells following Jackson have been reported, and, recently, several articles dealing with Si-H materials in stacked cells have been published. Marfaing proposed utilizing silane deposited amorphous Si-Ge alloys in stacked cells, but did not report the feasibility of doing so. (Y. Marfaing, Proc. 2nd European Communities Photovoltaic Solar Energy Conf., Berlin, West Germany, p. 287, (1979)).
Hamakawa et al., reported the feasibility of utilizing Si-H in a configuration which will be defined herein as a cascade type multiple cell. The cascade cell is hereinafter referred to as a multiple cell without a separation or insulating layer therebetween. Each of the cells was made of a Si-H material of the same band gap as in a p-i-n junction configuration. Matching of the short circuit current (J.sub.sc) was attempted by increasing the thickness of the cells in the serial light path. As expected, the overall device Voc increased and was proportional to the number of cells.
Due to the beneficial properties attained by the introduction of fluorine, amorphous alloys used to produce cascade type multiple cells may now incorporate fluorine as one possible method of reducing the density of localized states without impairing the electronic properties of the material. Further band gap adjusting element(s), such as germanium and carbon, can be activated and are added in vapor deposition, sputtering, or glow discharge processes. The band gap is adjusted as required for specific device applications by introducing the necessary amounts of one or more of the adjusting elements into the deposited alloy cells in at least the photocurrent generation region thereof. Since the band gap adjusting element(s) has been tailored into the cells without adding substantial deleterious states, because of the influence of fluorine, the cell material maintains high electronic qualities and photoconductivity when the adjusting element(s) are added to tailor the device wavelength characteristics for a specific photoresponsive application. The addition of hydrogen, either with fluorine or after deposition, can further enhance the fluorine compensated or altered alloy. The post deposition incorporation of hydrogen is advantageous when it is desired to utilize the higher deposition substrate temperatures allowed by fluorine.
It is of obvious commercial importance to be able to mass produce photovoltaic devices such as solar cells. However, with crystalline cells, mass production was limited to batch processing techniques by the inherent time consuming growth requirements of the crystals. Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon semiconductor alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for example, in pending U.S. Pat. Nos. 4,400,409, entitled "A Method of Making P-Doped Silicon Films and Devices Made Therefrom," 4,542,711 entitled "Continuous Systems For Depositing Amorphous Semiconductor Material," 4,410,558 entitled "Continuous Amorphous Solar Cell Production System," 4,438,723 entitled "Multiple Chamber Deposition and Isolation System and Method," 4,492,181 entitled "Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells," and 4,485,125, entitled "Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells." As disclosed in these patents, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor material. In making a solar cell of p-i-n-type configuration, the first chamber is dedicated for depositing a p-type amorphous silicon alloy, the second chamber is dedicated for depositing an intrinsic amorphous silicon alloy, and the third chamber is dedicated for depositing an n-type amorphous silicon alloy.
Since each deposited semiconductor alloy, and especially the intrinsic semiconductor alloy, must be of high purity; (1) the deposition environment in the intrinsic deposition chamber is isolated, by specially designed gas gates, from the doping constituents within the other chambers to prevent the diffusion of doping constituents into the intrinsic chamber; (2) the substrate is carefully cleansed prior to initiation of the deposition process to remove contaminants; (3) all of the chambers which combine to form the deposition apparatus are sealed and leak checked to prevent the influx of environmental contaminants; (4), the deposition apparatus is pumped down and flushed with a sweep gas to remove contaminants from the interior walls thereof; and (5) only the purest reaction gases are employed to form the deposited semiconductor materials. In other words, every possible precaution is taken to insure that the sanctity of the vacuum envelope formed by the various chambers of the deposition apparatus remains uncontaminated by impurities, regardless of origin.
The layers of semiconductor material thus deposited in the vacuum envelope of the deposition apparatus may be utilized to form a photovoltaic device including one or more p-i-n cells, one or more n-i-p cells, a Schottky barrier, photodiodes, phototransistors, or the like. Additionally, by making multiple passes through the succession of deposition chambers, or by providing an additional array of deposition chambers, multiple stacked cells of various configurations may be obtained.
As is obvious from the foregoing, thin film amorphous semiconductor materials offer several distinct advantages over crystalline materials, insofar as they can be easily and economically fabricated by the newly developed mass production processes. However, in the fabrication of semiconductor material by the aforementioned processes, the presence of current-shunting defects has been noted. These defects have (1) seriously impaired the performance of the photovoltaic devices fabricated therefrom and (2) detrimentally affected production yield. These process-related defects are thought to either (1) be present in the morphology of the substrate electrode, or (2) develop during the deposition of the semiconductor layers. It is to the end of eliminating, or at least substantially reducing the effects of these current-shunting defects to which the instant invention is directed.
The most important of these defects may be characterized as "shunt", "short-circuit" defects, defect regions or low resistance current paths. Before the suspected causes of these defects are explained, it is helpful to note the thicknesses of the deposited semi-conductor layers. A typical "p" layer may be only on the order of 400 angstroms thick, a typical "i" layer may be only on the order of 3,500 angstroms thick, and a typical "n" layer may be only on the order of 200 angstroms thick, thereby providing a total semiconductor body thickness of only about 4,100 angstroms. It should therefore be appreciated that irregularities, however small, are not easy to cover by the deposited semiconductor layers.
Shunt defects are present when one or more low resistance current paths develop between the electrodes of the photovoltaic device. Under operating conditions, a photovoltaic device in which a shunt defect has developed, exhibits either (1) a low power output, since electrical current collected at the electrodes thereof flows through the defect region (the path of least resistance) in preference to an external load, or (2) complete failure where sufficient current is shunted through the defect region to "burn out" the device.
While shunt-type defects always deleteriously affect the performance of photovoltaic devices, their effect is greatest when the devices in which they are incorporated are operated under relatively low illumination, such as room light, vis-a-vis, high intensity illumination such as AM 1. This occurs because, in a photovoltaic device, photo-generated current increases linearly with increasing illumination, while the resulting voltage increases exponentially with increasing illumination. In other words, voltage attains a relatively high value under low illumination, the value increasing only slightly as the intensity of the illumination is increased. The result is that under low illumination a relatively high voltage potential is present, said potential adapted to preferentially drive the relatively small number of current carriers through the path of least resistance, i.e., the low resistance defect regions. In contrast thereto, under high illumination, a large number of current carriers are present and are driven by a potential of about the same magnitude as the potential which exists under low illumination. This larger number of current carriers compete for a limited number of least resistance paths (through the defect regions). The result is that at high intensity, while about the same amount of power may be lost to the defect region, the power lost is a smaller percentage of the total power produced than at low intensity illumination.
Defects or defect regions, the terms being interchangeably used herein, are not limited to "overt" or "patent" short circuit current paths. In some cases, the adverse effects of a defect are latent and do not immediately manifest themselves. Latent defects can give rise to what will be referred to hereinafter as an "operational mode failure", wherein a photovoltaic device, initially exhibiting satisfactory electrical performance, suddenly fails. The failures will be referred to in this application as operational mode failures regardless of whether the device was previously connected to a load for the generation of power, it only being necessary that the device was, at some time, subjected to illumination, thereby initiating the generation of carriers. This type of failure will be discussed in further detail hereinbelow. It is believed the shunt defects, both latent and patent, arise from one or more irregularities in the (1) morphology of the substrate material, or (2) in the growth of the semiconductor layers.
The first, and perhaps most important, source of the defects, i.e., the aforementioned morphological irregularities in the deposition surface of the substrate material will now be discussed. Even though the highest quality stainless steel is employed to serve as the substrate or base electrode upon which the semiconductor layers are successively deposited, such substrates have been found to have as many as 10,000 irregularities per square centimeter on the deposition surfaces thereof. Such irregularities take the form of projections, craters, or other deviations from a smooth finish and may be under a micron in (1) depth below the surface, (2) height above the surface, or (3) diameter. Regardless of their configuration or size, the defect may establish a low resistance current path through the semiconductor body, thereby effectively short circuiting the two electrodes. This may occur in numerous ways. For instance, a spike projecting from the surface of the substrate electrode may be of too great a height to be covered by the subsequent deposition of semiconductor layers, and therefore, be in direct electrical contact with the other electrode when that electrode is deposited atop the semiconductor layers. Likewise, a crater formed in the surface of the substrate electrode may be of too small a size to be filled by the subsequent deposition of semiconductor layers and therefore, be in sufficient proximity to the other electrode, when that electrode is deposited atop the semiconductor layers, for electrical current to either (1) bridge the gap which exists between the electrodes, or (2) through actual use (the photoinduced generation of electrical current) of the photovoltaic device, cause the material of one of the electrodes to migrate toward and contact the other of the electrodes, and thereby pass electrical current therebetween. It is also possible that the semiconductor layers deposited onto the substrate are of poor quality, thereby providing a low resistance path for the flow of electrical current between the electrodes of the photovoltaic device.
Further, despite all the previously described efforts to maintain the vacuum envelope free of external contaminants; dust or other particular matter, which somehow either (1) invades the vacuum envelope during the deposition of the semiconductor material, or (2) forms as a by-product of the deposition process and is deposited over the substrate electrode along with the semiconductor material. The contaminants interfere with the uniform deposition of the semiconductor layers and may establish the low resistance current path through which electrical current either (1) directly communicates, or (2) may, through operation of the device, develop communication with the two electrodes. Additionally, it is suspected that in some cases, the semiconductor material may form micro-craters or micro-projections during the deposition thereof, even absent the presence of contaminants or pollutants from external sources. Again, such morphological deviation from a perfectly smooth and even surface means that the substrate is covered by semiconductor material either (1) in an "ultra thin layer" (consider again that the total thickness of all semiconductor layers is only on the order of 4,000 angstroms and any reduction in coverage is indeed an ultra thin layer) or (2) not at all. Obviously, when the upper electrode material is deposited across the entire surface of the semiconductor body, the defect regions cause the low resistance current path to develop, and electrical current is shunted therethrough. In still other cases involving defect regions, the presence of such defect regions is only detectable due to their deleterious effect upon the electrical and photoelectric properties of the resultant photovoltaic device. Finally, note the defects described hereinabove may not be sufficiently severe to divert all electrical current through the low resistance path. However, the diversion or shunting of any current therethrough represents a loss in operational efficiency of the photovoltaic device and should therefore be eliminated. Moreover, the shunting of even small amounts of current through each of thousands of defect regions may combine to cause major losses in efficiency. Based upon the foregoing, it should be apparent that a reduction in current flow through these defects and defect regions is critical to the fabrication of high-yield, high efficiency, large area thin film photovoltaic devices.
Several approaches dealing with gross amplifications of this problem have been implemented by Applicants and their colleagues. As described in U.S. Pat. No. 4,451,970, of Masatsugu Izu and Vincent Cannella, entitled "System and Method For Eliminating Short Circuit Current Paths In Photovoltaic Devices," said patent assigned to the assignee of the instant application, the shunting of current through defect regions is treated by substantially eliminating the defect regions as an operative area of the semiconductor device. This is accomplished in an electrolytic process where electrode material is removed from the periphery of the defect site, effectively isolating the defect region and preventing the flow of electrical current through the defect region. However, the process described in the aforementioned patent is current dependent, i.e., the greater the current flowing through a particular area of the device, such as a defect region, the greater the amount of electrode material (in the preferred embodiment indium tin oxide) is removed. Consequently, said short circuit eliminating process performs admirably in removing the electrode material from the periphery of a large defect, and thereby preventing all current flow therethrough. However, it is not as successful in eliminating the flow of current between the electrodes in the thousands of defect regions which are relatively small. And, as previously mentioned, since a great many of relatively small current shunting paths, taken in toto, divert a substantial amount of current from its desired path of travel through the semiconductor layers, the flow of current through the low resistance current paths created by such small defect regions must also be eliminated or at least substantially reduced. Further, the electrolytic process described in the '890 application neither detects nor helps in preventing the formation of current-shunting paths in the case of operational mode failures.
In U.S. Pat. No. 4,419,530 issued in the name of Prem Nath, entitled "Improved Solar Cell and Method For Producing Same", and assigned to the assignee of the instant patent application, there is described a method for electrically isolating small area segments of an amorphous, thin film, large area photovoltaic device. This is accomplished by (1) dividing the large area device into a plurality of small area segments, (2) testing the small area segments for electrical operability, and (3) electrically connecting only those small area segments exhibiting a predetermined level of electrical operability, whereby a large area photovoltaic device comprising only electrically operative small area segments is formed. One cause of the electrical inoperability (failure of the device to meet predetermined minimum specifications) of small area segments is the presence of defect regions which operate to shunt current through low resistance paths. The method of solving the problem of defect regions in the '530 patent, while effective, eliminates rather large sections of the photovoltaic device. The output of the device is significantly reduced as compared to the output which could be realized if only the precise defect region was eliminated.
While the method of dividing a relatively large area solar cell into electrically isolated portions and thereafter only connecting those portions which have satisfactory output is effective in reducing or eliminating the effect of defects, this concept is not completely satisfactory for several reasons.
The step of dividing the semiconductor body of the solar cell into electrically isolated portions requires several production steps and also reduces the total area of the solar cell that is available for producing electrical energy. Further, the method can be time and cost intensive since the electrical output of each isolated portion is tested and then separate electrical connections are made to an electrical contact for each small area to transmit the current produced by the device. In addition, since it is possible that defects (shorts) in a solar cell can develop after the cell has been in use, the concept of dividing the body of the large area cell does not make compensation for this type of defect.
Further, both of the foregoing patent applications relate to "after market" techniques which are applicable to (1) isolate only gross defect-containing regions of a photovoltaic device, and (2) prevent any and all current flow through those defect containing regions. Accordingly, a need still exists for a photovoltaic device which substantially eliminates the deleterious effect or shunt defects, both large and small, whatever their origin, without operatively removing large portions of the active semiconductor body, and while maintaining an acceptable level of current flow across the entire surface of the device.
In order to more efficiently make and utilize large area photovoltaic devices, it would be desirable to provide an improved solar cell and method in which the effects of any shunt defects (shorts) in the cell, either when fabricated or which later develop after use, are minimized without the necessity of dividing a large area solar cell into a plurality of electrically isolated portions. In addition, a need exists for an improved solar cell which can be more easily fabricated on a large scale. Finally, a need exists for improvements in collecting current generated by solar cells so that the current is uniformly collected across the area of the cell.